1. Field of the Invention
This invention relates to a method of controlling a brushless motor. More particularly, the present invention relates to a technique that can be effectively applied to a brushless motor to be used for an electric power steering apparatus or an electronically controlled throttle valve.
2. Related Art Statement
Torque ripples have been an important problem that needs to be resolved. Torque ripples have to be reduced in order to achieve a low vibration level and a low noise emission level for brushless motors. For example, the motors that are used in electric power steering apparatuses are required to reduce the torque ripple because it adversely affects the feeling of the driver steering the automobile to a large extent. Similarly, the motors that are used in electronically controlled throttle valves of engines are required to reduce the torque ripple from the viewpoint of engine control because it influences the opening/closing operations of the throttle valve from the viewpoint of responsiveness. This problem is serious particularly when the valve is closing. Therefore, in such motors, the rotary position of the rotor is precisely detected by means of a resolver and the motor is made to revolve smoothly with a small torque ripple by means of sinusoidal wave drive.
However, when a resolver is used as unit in a brushless motor, it is expensive and a dedicated R/D converter (resolver signal/digital signal converter) has to be installed as interface with the corresponding control unit. Therefore, a motor using a resolver is accompanied by a problem of high cost of the entire system.
Thus, there is a demand for a system with a reduced torque ripple that can be obtained by using less expensive magnetic detection elements such as Hall elements as in the case of ordinary brushless motors so as to avoid the use of a resolver and an R/D converter. However, in the case of conventional systems for driving 3-phase brushless motors by using three magnetic detection elements, the accuracy of detection of the rotor position is poor and a rectangular drive mode has to be employed so that consequently it is difficult to reduce the torque ripple.
Thus, so-called overlapping energization is used in a brushless motor that is required to show a low vibration level and a low noise emission level as means for reducing the torque ripple while relying on 120° rectangular wave drive. Overlapping energization is a mode of electric energization in which a plurality of phases that are excited in the +or −direction are provided in an overlapping fashion for commutation. For example, when the U-phase is switched to the V-phase in a 3-phase motor, there is provided a time period during which the two phases are simultaneously energized to the same polarity (+or −). More specifically, in the above example, the energization of the V-phase in the +direction is started before the energization of the U-phase in the +direction is stopped so that the adjacent two phases are excited to the same polarity in an overlapping fashion in order to commutate from the U-phase to the V-phase.
With overlapping energization, the extent of overlapping is determined by the timing of energization of the next phase. The timing of energization of the next phase is by turn determined by estimating the rotor position by means of a timer or a piece of software on the basis of the information on the rotary position of the rotor obtained by the magnetic detection elements. With this arrangement, two phases are appropriately made to overlap each other and excited so that a phase is smoothly switched to another and it is possible to realize pseudo-sinusoidal wave drive. Therefore, it is possible to reduce the torque ripple by means of less costly magnetic detection elements without relying on a resolver and an R/D converter that are expensive.
However, in the case of an electric power steering apparatus or an electronically controlled throttle valve, forward revolutions and backward revolutions are switched from one to the other fiercely as the steering apparatus is operated or the throttle valve is opened or closed and the resultant acceleration changes greatly. Additionally, the steering operation and the accelerating operation can remarkably differ in individuals and some can steer and accelerate the car in a highly abrupt manner. Therefore, it is difficult to accurately predict the behavior of the motor. It is also difficult to predict the behavior of the motor when the motor speed is low and/or the motor is being accelerated. Thus, the rotary position of the rotor estimated by means of a timer or a piece of software can involve a large error and hence the overlapping energization cannot be conducted at appropriate timings.
In other words, overlapping energization is effective for motors that are being driven in a constant drive condition where the direction of revolution, the number of revolutions per unit time and the degree of acceleration scarcely change. However, it is difficult to adopt overlapping energization for electric power steering apparatuses and electronically controlled throttle valves. Therefore, a highly expensive resolver has to be used for such motors to raise the cost of the motor.